The present invention relates to a process for the production of vinyl chloride polymers by discontinuous emulsion polymerization in the presence of water-soluble catalysts and emulsifiers, as well as polymerization adjuvants.
It has been known to manufacture polyvinyl chloride suitable for paste preparation by continuous or discontinuous processes.
The continuous process produces a PVC paste having a desirably low viscosity in the high shear range, but with a high viscosity at low shear rates. In addition, such polyvinyl chlorides contain a relatively high proportion of emulsifiers. A high emulsifier concentration has an adverse effect on the final products and during processing. For example, it will impair transparency and increase the water sensitivity of the polymers and of the articles produced therefrom.
In the discontinuous process, two versions are known in principle. According to the so-called microsuspension process, latices are obtained having a relatively low solids content of about 40% by weight. Spray-drying of these latices thus results in increased drying costs. In addition, considerable expenditures are connected with the conductance of the process since it requires an additional step preceding the polymerization, namely a preliminary homogenizing of water, vinyl chloride, emulsifiers, as well as monomer-soluble catalysts using homogenizers. The spray-dryed products result in pastes having pronounced dilatant flow behavior. Accordingly, they have little suitability for use in spread-coating methods.
French Pat. No. 2,086,635 discloses a two-stage miscrosuspension process describing the production of PVC latices having high solids concentrations. However, in addition to the additional preliminary homogenization, the method demands the preparation of a seed latex and thus entails further excessive expenditures.
By using the discontinuous version of vinyl chloride emulsion polymerization, polymers are obtained having low emulsifier contents. In order to obtain stable latices and polyvinyl chloride pastes having a low paste viscosity when conducting the polymerization with low emulsifier quantities, it has been suggested to utilize the so-called seed latex method. Therein, a seed latex (nucleating latex or primary dispersion) is provided for the emulsion polymerization. However, this operation is very expensive on a large industrial scale, since the primary dispersion must be separately prepared, stored, and dosed. Furthermore, the attainable stability of the thus-prepared latices is not optimized. The viscosities of the polyvinyl chloride pastes obtained according to this method still lie markedly above the paste viscosities achieved according to the continuous process.
In accordance with the methods described in German Pat. No. 1,964,029 and the laid-open text of Belgian Pat. No. 656,985, the emulsifier is added during the polymerization procedure in accordance with a specific program. It is possible by these methods to prepare polyvinyl chloride having especially low emulsifier contents.
However, the polyvinyl chloride pastes obtained according to this process still exhibit a substantially higher viscosity than those obtained by using the continuous procedure (see, in this connection, Comparative Example D herein).
Discontinuous processes are known which are conducted using dispersion aids (fatty alcohols); see, e.g.,
1. DOS No. 2,260,957 PA0 2. DAS No. 2,629,655 PA0 3. DOS No. 2,850,105 PA0 4. DOS No. 2,742,178.
The polymerizations of (1), (2), and (3) are effected by means of oil-soluble activators (catalysts), partially with the addition of reducing agents. These processes lead to products which, when processed into plastisols, show a strongly dilatant flow characteristic, as can be seen from Comparative Experiment A herein.
The process described in (4) uses inorganic catalysts for activating purposes. According to (4), a preliminarily agitated mixture consisting of a C.sub.16 -C.sub.20 -alkyl alcohol and an alkyl sulfate (C.sub.12 -C.sub.18) is employed in the polymerization. As demonstrated by Comparative Experiments B and C herein, either unstable latices or low-solid-content latices are obtained according to the method of (4). After spray-drying, these yield a polyvinyl chloride resulting in highly viscous pastes having a pronounced pseudo-plastic flow behavior.